Semiconductor nanocrystals (also known as quantum dot particles) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of semiconductor nanocrystals shift to the blue (higher energies) as the size of the nanocrystals gets smaller.
Semiconductor nanocrystals are nanoparticles composed of an inorganic, crystalline semiconductive material and have unique photophysical, photochemical and nonlinear optical properties arising from quantum size effects, and have therefore attracted a great deal of attention for their potential applicability in a variety of contexts, e.g., as detectable labels in biological applications, and as useful materials in the areas of photocatalysis, charge transfer devices, and analytical chemistry. As a result of the increasing interest in semiconductor nanocrystals, there is now a fairly substantial body of literature pertaining to methods for manufacturing such nanocrystals.
In general, these routes can be classified as involving preparation in glasses (Ekimov et al., JETP Letters 34:345 (1981)); aqueous preparation, including preparations that involve use of inverse micelles, zeolites, Langmuir-Blodgett films, and chelating polymers (Fendler et al., J. Chem. Society, Chemical Communications 90:90 (1984) and Henglein et al., Ber. Bunsenges. Phys. Chem. 88:969 (1984)); and high temperature pyrolysis of organometallic semiconductor precursor materials, i.e., rapid injection of precursors into a hot coordinating solvent (Murray et al., J. Am. Chem. Soc. 115:8706 (1993) and Katari et al., J. Phys. Chem. 98:4109 (1994)). The two former methods yield particles that have unacceptably low quantum yields for most applications, a high degree of polydispersity, poor colloidal stability, a high degree of internal defects, and poorly passivated surface trap sites. In addition, nanocrystals made by the first route are physically confined to a glass matrix and cannot be further processed after synthesis.
Improved synthesis conditions have been reported that utilize cadmium salts (Peng, et al., J. Am. Chem. Soc. 123:183–184 (2001)). These conditions provide certain advantages over the rapid injection method. The use of cadmium acetate, cadmium oxide or other such Cd(II) salts, pre-complexed with a ligand such as tetradecylphosphonic acid provides for a cadmium precursor that is particularly suitable for nanocrystal synthesis. These reactions have numerous desirable features, including improved safety and relatively wide tolerance for production variables such as precursor injection rate and temperature. Of particular note is that these reactions can be tuned to yield very narrow photoluminescence spectra over a wide range of useful wavelengths. Unfortunately, it is difficult to optimize the particle yield, while maintaining the desirable features of the Cd(II) synthesis conditions. In particular, for smaller size nanoparticle synthesis, yields have been very poor under Cd(II) synthesis conditions. Reaction conditions that provide such low yields are not only more expensive to implement on a manufacturing scale, but they often require much larger reactors and produce more hazardous waste.
Thus, there remains a need in the art for improved methods for manufacturing nanoparticles, and smaller nanoparticles in particular. Such methods would ideally provide a high product yield of internally defect free, high band edge luminescence nanoparticles with no or minimal trapped emission. Such methods would also ideally provide for the manufacture of particles that exhibit near monodispersity and have a relatively narrow particle size distribution. Finally, such methods would be useful not only with semiconductor nanoparticles, but also with other types of nanoparticles, e.g., semiconductive nanoparticles that are not necessarily crystalline and metallic nanoparticles.
The present invention addresses those needs by providing improved methods for manufacturing nanoparticles. By controlling the nucleation density the methods of the invention provide for a predictable and controllable final particle size, as well as many of the aforementioned properties.